Heitstrenging (pl. heitstrengingar) is an Old Norse practice of swearing of a solemn oath to perform a future action. They were often performed at Yule and other large social events, where they played a role in establishing and maintaining good relationships principally between members of the aristocratic warrior elite. The oath-swearing practice varied significantly, sometimes involving ritualised drinking or placing hands on a holy pig (referred to as a sónargöltr) that could later be sacrificed. While originally containing heathen religious components such as prayers and worship of gods such as Freyr and Thor, the practice continued in an altered manner after the Christianisation of Scandinavia.

Close similarities have been noted between heitstrengingar and Anglo-Saxon oath-swearing and boasting practices such as the beot, with both involving promises to accomplish tasks that would bring great honour if achieved.

In Old Norse, the term heitstrenging or heit-strenging ('the swearing of a solemn vow') is derived from the verb 'heit-strengja' ('to vow'), in turn formed from the joining of heit ('a solemn vow') and the verb strengja ('to string', 'to fasten').

Heitstrenging at Yule-evening is described in the prose of Helgakviða Hjörvarðssonar, involving placing one's hands on the bristles of the sónargöltr and drinking of the bragarfull:

Hervarar saga ok Heiðreks describes heitstrengingjar taking place in a similar manner to in Helgakviða Hjǫrvarðssonar. The principal manuscripts for the saga vary considerably with the account in manuscript H as follows:

Here, the ritual is dedicated to only Freyr, while in the U manuscript it is also to Freyja though this is typically amended to Freyr. The U manuscript furthermore describes the ritual and the involvement of the boar to be a custom at the time of the saga narrative and tells how there was an effort to breed the largest pig possible for the occasion.

In Fagrskinna, the term heitstrenging is used to describe the swearing of an oath on a bragarfull at Harald Bluetooth's memorial feast (Old Norse: veizluerfi):

Later in the account, Sweyn Forkbeard swears on the bragafull to conquer England or die in the attempt, before claiming his inheritance, and Earl Sigvaldi does the same for Norway.